Motor-temperature estimation apparatus and motor control apparatus

ABSTRACT

A motor-current square value calculation section  45  calculates, as a motor-current square value, the sum of respective square values of two phase currents converted from three phase currents flowing through an electric motor. A temperature estimation calculation section  44  performs low-pass filtering for the motor-current square value, and calculates temperature increases of the mass portion and coils on the basis of the filtered motor-current square value. In the low-pass filtering, filtering calculation is performed in a manner which changes depending on rotational speed RV of the electric motor; i.e., whether the electric motor is in a rotating state or in a stopped state. The temperature increase of the mass portion is added to ambient temperature of the electric motor to thereby estimate temperature of the mass portion, and the temperature increase of the coils is added to the estimated temperature of the mass portion to thereby estimate temperature of the coils. Accordingly, the coil temperature of the electric motor can be accurately detected, and current flowing through the electric motor is properly controlled in accordance with the detected temperature.

TECHNICAL FIELD

The present invention relates to a motor-temperature estimationapparatus for estimating temperature of the housing, stator, coils, orthe like of an electric motor, and to a motor control apparatus capableof limiting current flowing through an electric motor in accordance withcoil temperature of the motor estimated by use of the motor-temperatureestimation apparatus.

BACKGROUND ART

As shown in Japanese Patent Application Laid-Open (kokai) No. H10-67335,a conventional apparatus includes a temperature sensor provided withinan electric motor, and corrects drive current on the basis oftemperature detected by use of the temperature sensor, to therebycompensate drop in permeability of the stator of the motor stemming fromtemperature increase. Further, in the publication, there is proposed atechnique of estimating temperature of an electric motor throughintegration of drive current flowing through the motor.

The above-described technique which uses a temperature sensorincorporated in an electric motor is not preferable from the viewpointof cost and easiness of mounting of the motor onto an apparatus whichuses the motor, because of the necessity of providing a temperaturesensor and connection wires for connecting the temperature sensor and acontrol circuit of the motor. Although the above-mentioned patentpublication proposes a technique of estimating temperature of anelectric motor through integration of drive current flowing through themotor, it does not describe the details of the technique.

DISCLOSURE OF THE INVENTION

The present invention has been accomplished in order to cope with theabove-described problem, and an object of the present invention is toprovide a motor-temperature estimation apparatus which can accuratelydetect temperature of the housing, stator, coils, or the like of anelectric motor, without the necessity of incorporating a temperaturesensor into the motor. Another object of the present invention is toprovide a motor control apparatus which can limit current flowingthrough an electric motor in accordance with coil temperature of themotor estimated by use of the motor-temperature estimation apparatus, tothereby reliably suppress temperature rise of the coils of the motor.

In order to achieve the above object, the present invention includes, ascharacteristic features, current detection means for detecting currentflowing through a coil of an electric motor; determination means fordetermining whether the electric motor is in a rotating state or in astopped state; and estimation calculation means for estimatinglycalculating temperature of the electric motor on the basis of thedetected current and through calculation which changes depending onresults of the determination as to whether the electric motor is in therotating state or in the stopped state. In this case, the temperature ofthe electric motor refers to, for example, temperature of a housing,temperature of a stator, or temperature of the coil.

The estimation calculation means preferably includes square valuecalculation means for calculating a square value of the detectedcurrent; low-pass-filtering means for performing low-pass-filteringcalculation for the calculated square value in a manner which changesdepending on results of the determination as to whether the electricmotor is in the rotating state or in the stopped state; temperatureincrease calculation means for calculating a temperature increasestemming from current flowing through the coil on the basis of thelow-pass-filtered square value; and temperature calculation means forcalculating temperature of the electric motor on the basis of thecalculated temperature increase.

More specifically, the low-pass filtering delays (in other words,smoothes) change in the current square value (a parameter correspondingto quantity of generated heat). Preferably, when the electric motor isin the stopped state, change in the current square value is renderedsharp; i.e., delay of change is reduced, as compared with the case wherethe electric motor is in the rotating state. Further, the temperaturecalculation means may calculate temperature of the electric motorthrough addition, to ambient temperature of the electric motor, of thetemperature increase calculated by means of the temperature increasecalculation means.

In the case where such a motor temperature estimation apparatus isapplied to a three-phase motor, the motor temperature estimationapparatus may be configured in such a manner that the current detectionmeans detects two phase currents converted from three phase currents;and the estimation calculation means estimatingly calculates temperatureof the electric motor on the basis of the sum of respective squarevalues of the two phase currents. Alternatively, the motor temperatureestimation apparatus may be configured in such a manner that the currentdetection means detects three phase currents, and the estimationcalculation means estimatingly calculates temperature of the electricmotor on the basis of respective square values of the three phasecurrents.

Under the present invention having the above-described structuralfeatures, temperature of the electric motor is estimated throughcalculation which changes depending on results of the determination bythe determination means as to whether the electric motor is in therotating state or in the stopped state. In an electric motor,temperature increase caused by current flowing through the coil in astopped state (rotation restrained state) is greater than temperatureincrease caused by current flowing through the coil in a rotating state.Therefore, according to the present invention, the difference intemperature increase caused by current flowing through the coil betweenthe rotating state and the stopped state is taken into consideration,and thus estimation of temperature of the electric motor can beperformed accurately. As a result, without necessity of incorporating atemperature sensor in the electric motor, various controls on the basisof temperature of the electric motor, such as limiting of coil currentand correction of decrease in permeability of the stator caused bytemperature increase, can be performed by means of a simpleconfiguration.

Another feature of the present invention resides in a motor controlapparatus comprising current limiting means for limiting current flowingthrough the electric motor in accordance with the coil temperatureestimated by means of the motor temperature estimation apparatus, whichis configured as described above. By virtue of this configuration,current flowing through the electric motor is limited in accordance withthe coil temperature detected with high accuracy, whereby temperatureincrease of the coil of the electric motor can be securely suppressed.

In this case, the current limit means may be configured to limit currentflowing through the electric motor to a predetermined limit value orless, when the estimated coil temperature exceeds a predeterminedtemperature. By virtue of this configuration, current flowing throughthe electric motor is forcedly limited to the limit value or less,whereby temperature increase of the coil of the electric motor can bereliably suppressed.

Moreover, when current flowing through the electric motor is limited tothe limit value or less, preferably, the limit value is graduallychanged. By virtue of this configuration, even when coil temperaturechanges abruptly, current flowing through the electric motor changesgradually, whereby abrupt change in output of the electric motor can beavoided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the entire configuration of anelectric power steering apparatus of a vehicle to which amotor-temperature estimation apparatus and a motor control apparatusaccording to an embodiment of the present invention are applied.

FIG. 2 is a functional block diagram showing, in detail, a motoroverheat prevention control section of FIG. 1.

FIG. 3 is a time chart showing on-off operation of an ignition switchand the overall operation flow of the motor overheat prevention controlsection.

FIG. 4 is a graph showing the relation between coil estimationtemperature TPco and wait time.

FIG. 5A is a graph showing a change in mass-temperature initialcorrection coefficient Ktpm with printed board temperature change ΔTPbd.

FIG. 5B is a graph showing a change in coil-temperature initialcorrection coefficient Ktpc with printed board temperature change ΔTPbd.

FIG. 6 is a flowchart of a motor-temperature estimation program executedin a temperature estimation calculation section.

FIG. 7 is a time chart showing changes with time in motor-current squarevalue Ismt, current square value for mass Isma, and current square valuefor coils Isco.

FIG. 8 is a graph showing the relation between coil estimationtemperature TPco and first current limit value IL1.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described with referenceto the drawings. FIG. 1 schematically shows the entire configuration ofan electric power steering apparatus of a vehicle to which amotor-temperature estimation apparatus and a motor control apparatusaccording to the present invention are applied.

First, the electric power steering apparatus will be described briefly.The electric power steering apparatus includes, as an electric motor, abrushless motor 11, which is a three-phase synchronous permanent magnetmotor. The brushless motor 11 includes a stator fixedly disposed withina housing, and forms a three-phase rotating magnetic field throughsupply of three-phase current to coils wound on the stator, whereby arotor to which permanent magnets are fixed rotates in the three-phaserotating magnetic field in accordance with the three-phase current.

Upon rotation of the rotor, the brushless motor 11 imparts assist forceto steering motion of front wheels generated by rotating operation of asteering wheel 12. Specifically, a rack bar 14 is connected to thesteering wheel 12 via a steering shaft 13 and a pinion gear (not shown),so that the rack bar 14 is displaced in an axial direction with rotationof the steering wheel 12. The rack bar 14 is also driven in the axialdirection upon rotation of the brushless motor 11. The left and rightfront wheels are steerably connected to the opposite ends of the rackbar 14 via tie rods and knuckle arms (both are not shown).

A torque sensor 15 is attached to a lower end portion of the steeringshaft 13, and detects steering torque TR imparted to the steering shaft13. Moreover, a rotational angle sensor 16 formed by an encoder isattached to the brushless motor 11 in order to detect motor rotationalangle (mechanical angle) θm. Upon rotation of the rotor of the brushlessmotor 11, the rotational angle sensor 16 outputs two-phase pulse-trainsignals having a phase difference of π/2, and a zero-phase pulse-trainsignal representing a reference rotational position. Data indicatingtorque TR detected by means of the steering toque sensor 15 and motorrotational angle θm detected by means of the rotational angle sensor 16are supplied to an electronic control circuit unit ECU (indicated by achain-line frame in FIG. 1). Moreover, data indicating vehicle speed Vdetected by means of a vehicle speed sensor 17 is supplied to theelectronic control circuit unit ECU.

The electronic control circuit unit ECU controls three-phase current(assist current) supplied to the coils of the brushless motor 11, andincludes an assist-current calculation section 21 for receiving the dataindicating vehicle speed V and steering torque TR. The assist-currentcalculation section 21 calculates two instruction phase currents Id* andIq* corresponding to assist torque which increases with steering torqueTR and decreases with increase of vehicle speed V. Notably, these twoinstruction phase currents Id* and Iq* correspond to a d-axis and aq-axis, respectively, of a rotating coordinate system which issynchronous with rotating magnetic flux produced by a permanent magneton the rotor of the brushless motor 11, where the d-axis extends in thesame direction as that of the permanent magnet, and the q-axis extendsin a direction perpendicular thereto. In the present embodiment, theinstruction current Id* is set to “0.” These two instruction phasecurrents Id* and Iq* are supplied to a feedback control section 23 via acurrent limit section 22, which will be described in detail later.

Two phase currents Id and Iq, which are converted from three phasecurrents Iu, Iv, and Iw flowing through the coils of the brushless motor11, are supplied to the feedback control section 23. A current sensor 24detects the three phase currents Iu, Iv, and Iw. A 3-phase/2-phaseconversion section 25 converts the three phase currents Iu, Iv, and Iwto the two phase currents Id and Iq. For the purpose of 3-phase/2-phaseconversion, a rotational angle conversion section 26, which convertsmotor rotational angle θm to electrical angle θe, is connected to the3-phase/2-phase conversion section 25. The feedback control section 23produces difference signals Id*−Id and Iq*−Iq, which representrespective differences between the two instruction phase currents Id*and Iq* and the two phase currents Id and Iq, so as to feedback-controlthe three phase currents Iu, Iv, and Iw flowing through the coils of thebrushless motor 11.

The two phase difference signals Id*−Id and Iq*−Iq are converted tothree phase signals by means of a 2-phase/3-phase conversion section 27;and the thus-obtained three phase signals are supplied to a PWM controlcircuit 28. For the purpose of the 2-phase/3-phase conversion, theelectrical angle θe output from the rotational angle conversion section26 is supplied to the 2-phase/3-phase conversion section 27. On thebasis of the three phase signals output from the 2-phase/3-phaseconversion section 27, the PWM control circuit 28 produces pulse-widthmodulation (PWM) control signals corresponding to the difference signalsId*−Id and Iq*−Iq, and supplies them to a drive circuit 31.

The drive circuit 31 switches battery voltage Ebt output from a battery32 in accordance with the PWM control signals so as to supply threephase currents Iu, Iv, and Iw, which correspond to the PWM controlsignals, to the coils of the brushless motor 11 via an electromagneticrelay 33. The electromagnetic relay 33 is controlled by means of a relaycontrol circuit 34, which will be described later in detail, and isusually in an on state after an ignition switch 35 is turned on.Therefore, during periods in which steering assist by the brushlessmotor 11 is effected, normally, the PWM-controlled three phase currentsIu, Iv, and Iw flow through the coils of the brushless motor 11.

By virtue of operations of the above-described respective circuits,three phase currents Iu, Iv, and Iw corresponding to the steering torqueTR and the vehicle speed V are caused to flow through the brushlessmotor 11. Accordingly, the brushless motor 11 imparts assist torquecorresponding to the steering torque TR and the vehicle speed V forsteering operation of the steering wheel 12 by a driver. Notably, theabove-described assist-current calculation section 21, current limitsection 22, feedback control section 23, 3-phase/2-phase conversionsection 25, rotational angle conversion section 26, and 2-phase/3-phaseconversion section 27 within the electronic control circuit unit ECU maybe formed by hardware; however, in the present embodiment, thesesections are realized through execution of a microcomputer program. Inother words, these sections 21 to 23 and 25 to 27 represent, in the formof a block diagram, various functions realized through execution of themicrocomputer program.

Next, there will be described a motor overheat prevention apparatuswhich is applied to the above-described electric power steeringapparatus for a vehicle and which relates directly to the presentinvention. The motor overheat prevention apparatus includes a motoroverheat prevention control section 40, which is disposed in theelectronic control circuit unit ECU. The motor overheat preventioncontrol section 40 estimates temperatures of the housing, stator, andcoils of the brushless motor 11, and limits current flowing through thebrushless motor 11 when the coil temperature is high.

An ambient temperature sensor 51, a board temperature sensor 52, a speedconversion section 53, and a potentiometer 54 are connected to the inputside of the motor overheat prevention control section 40. The ambienttemperature sensor 51 is attached to the steering torque sensor 15, anddetects ambient temperature TPtr of a location where the brushless motor11 is disposed (outside air temperature). Notably, the ambienttemperature sensor 51 is not necessarily required to be attached to thesteering torque sensor 15, and may detect temperature at an arbitrarylocation near the brushless motor 11, so long as the ambient temperaturesensor 51 can detect temperature at a location other than locations atwhich temperature increases due to influence of current flowing throughthe motor 11.

The board temperature sensor 52 is attached to a printed circuit board,on which the electronic control circuit unit ECU is disposed, anddetects printed board temperature TPbd. The printed board temperatureTPbd is used to estimate temperature drop which relates to ambienttemperature and time between stop of operation of the brushless motor 11and resumption thereof. Therefore, a temperature sensor for detectingtemperature of a component other than the printed circuit board may beused instead of the board temperature sensor 52, so long as thealternative temperature sensor can detect temperature of a portion whosetemperature increases due to influence of current flowing through themotor 11. For example, a temperature sensor for detecting temperature ofan element which partially constitutes the electronic control circuitunit ECU may be used in place of the board temperature sensor 52.

The speed conversion section 53 differentiates the electrical angle θeoutput from the rotational angle conversion section 26 to thereby outputa signal indicative of rotational speed RV of the rotor of the brushlessmotor 11. The potentiometer 54 is connected to the battery 32 via theignition switch 35, and outputs, for detection of on-off state of theignition switch 35, a voltage corresponding to the battery voltage Ebtsupplied to the potentiometer via the ignition switch 35.

The current limit section 22, the relay control circuit 34, and a powersupply circuit 55 are connected to the output side of the motor overheatprevention control section 40. The current limit section 22 receivesmotor current limit value ILm from the motor overheat prevention controlsection 40, and limits the instruction current Iq* (Id* is “0”),calculated at the assist-current calculation section 21, to the motorcurrent limit value ILm or less, whereby current flowing through thecoils of the brushless motor 11 is limited so as not to exceed the motorcurrent limit value ILm. Notably, in actuality, three phase currents Iu,Iv, and Iw flow through the brushless motor 11; therefore, the threephase currents Iu, Iv, and Iw are limited so as not to exceedcorresponding phase current values which are converted from the motorcurrent limit value ILm.

The relay control circuit 34 turns the electromagnetic relay 33 on andoff under the control of the motor overheat prevention control section40, the electromagnetic relay 33 being provided in a current path to thebrushless motor 11. The power supply circuit 55 is connected to thebattery 32. Under the control of the motor overheat prevention controlsection 40, the power supply circuit 55 steps the voltage from thebattery 32 up or down so as to supply operation voltages for variouscontrol sections and circuits.

Moreover, a writable, nonvolatile memory (hereinafter referred to asEEPROM) 60 is connected to the motor overheat prevention control section40. The EEPROM 60 is used to store, at the end of operation of thebrushless motor 11, various parameters which are needed to estimatetemperatures of the housing, stator, and coils of the brushless motor 11while the motor is in operation. Notably, the motor overheat preventioncontrol section 40 and the speed conversion section 53 may be formed byhardware; however, in the present embodiment, these sections arerealized through execution of a microcomputer program. In other words,these sections 40 and 53 represent, in the form of a block diagram,various functions realized through execution of the microcomputerprogram.

Next, the motor overheat prevention control section 40 will be describedin detail with reference to the functional block diagram of FIG. 2. Inthis case as well, respective sections mentioned in the followingdescription represent, in the form of a block diagram, various functionrealized through execution of the microcomputer program. In thefollowing description, unless otherwise specified, it is assumed thatvoltage needed for operation is supplied from the power supply circuit55 to the respective sections and circuits, and that the electromagneticrelay 33 is in the on state, so that three phase currents Iu, Iv, and Iwflow from the drive circuit 31 to the coils of the brushless motor 11.

Before motor overheat prevention operation is specifically described,on-off operation of the ignition switch 35 and the overall operationflow of the motor overheat prevention control section 40 are describedwith reference to the time chart of FIG. 3. When the ignition switch 35is turned on at time t0, an ignition-on detection section 41 a detectsthe turn-on operation of the ignition switch 35 from an increase in thebattery voltage Ebt output from the potentiometer 54. Notably, thebattery voltage Ebt output from the potentiometer 54 undergoesanalog-to-digital conversion performed by means of an unillustrated A/Dconverter. In response to detection of the turn-on operation of theignition switch 35, the motor overheat prevention control section 40performs unillustrated initial check processing. At predetermined timet1 during the initial check, a data read-write processing section 42reads various stored data from the EEPROM 60, and supplies them to aninitial temperature correction calculation section 43.

The initial temperature correction calculation section 43 calculatesinitial values of mass estimation temperature TPma, which is a variablerepresenting estimated temperature of the mass portion (corresponding tothe housing and stator) of the brushless motor 11, and coil estimationtemperature TPco, which is a variable representing estimated temperatureof the coils. A temperature estimation calculation section 44 starts toestimate, at predetermined short intervals, values of mass estimationtemperature TPma and coil estimation temperature TPco by making use ofthese initial values and a motor current square value Ismt calculated ata motor-current square value calculation section 45. At time t2, a firstcurrent limit value calculation section 46 a, a second current limitvalue calculation section 46 b, and a minimum-value selection section 46c start control for limiting current flowing through the brushless motor11 on the basis of the value of coil estimation temperature TPco.Notably, initial value calculation and estimation calculation for massestimation temperature TPma and coil estimation temperature TPco, andoperation for limiting the current of the brushless motor 11 will bedescribed later in detail.

Immediately after start of control of the brushless motor 11(immediately after time t2), the data read-write processing section 42writes predetermined large values in the EEPROM 60 as tentative data ofrespective storage values TPtrep, TPmaep, TPcoep of ambient temperatureTPtr, mass estimation temperature TPma, and coil estimation temperatureTPco, in preparation for the case where data writing after that time isimpossible. The reason why the large values are used is to avoid thepossibility that in future estimation, ambient temperature TPtr and massestimation temperature TPma are calculated as temperatures lower thanactual temperatures.

When the ignition switch 35 is turned off at time t3 during control ofthe brushless motor 11, an ignition-off detection section 41 b detectsthe turn-off operation of the ignition switch 35 from a decrease in thebattery voltage Ebt output from the potentiometer 54. In this case, theignition-off detection section 41 b detects the turn-off of the ignitionswitch 35 upon satisfaction of conditions that the battery voltage Ebtis lower than a predetermined value and that this low voltage statecontinues for at least a predetermined period.

In response to detection of the turn-off of the ignition switch 35, themotor overheat prevention control section 40 performs various controls(IG off control) in relation to the turn-off of the ignition switch 35.As the ignition off control (IG off control), the data read-writeprocessing section 42 writes, immediately after the detection of theturn-off of the ignition switch 35 (immediately after time t3), thepresent values of mass estimation temperature TPma and coil estimationtemperature TPco in the EEPROM 60 as mass temperature storage valueTPmaep and coil temperature storage value TPcoep. This processing isperformed in preparation for the case in which writing of data in theEEPROM 60 becomes impossible at the time of power off.

Moreover, in response to detection of turn-off of the ignition switch35, a power-off control section 47 sets a wait time before power off. Inthis wait-time setting operation, a wait-time table stored in thepower-off control section 47 is referred to, and a wait timecorresponding to coil estimation temperature TPco at that time is set.As shown in FIG. 4, the wait time table stores wait time which increaseswith coil estimation temperature TPco.

At time t4 at which the above-mentioned IG off control processing ends,the power-off control section 47 starts measurement of the wait time setas described above, and outputs to the relay control circuit 34 a relayoff signal Roff for turning the electromagnetic relay 33 off. As aresult, the relay control circuit 34 turns the electromagnetic relay 33off, whereby no current flows through the brushless motor 11 after thattime. This operation is performed so as to minimize current flowingwithin the electronic control circuit unit ECU.

When the power-off control section 47 completes the measurement of theset wait time, at time t5, the power-off control section 47 instructsthe data read-write processing section 42 to write data. The dataread-write processing section 42 writes ambient temperature TPtr andprinted board temperature TPbd at that time in the EEPROM 60 as ambienttemperature storage value TPtrep and printed board temperature storagevalue TPbdep, respectively. Moreover, the data read-write processingsection 42 writes the difference TPma-TPtr between mass estimationtemperature TPma and ambient temperature TPtr at that time and thedifference TPco-TPma between coil estimation temperature TPco and massestimation temperature TPma at that time in the EEPROM 60 asmass-ambient temperature difference storage value ΔTPmtep and coil-masstemperature difference storage value ΔTPcmep, respectively.

After the above data are written in the EEPROM 60 as ambient temperaturestorage value TPtrep, printed board temperature storage value TPbdep,mass-ambient temperature difference storage value ΔTPmtep, and coil-masstemperature difference storage value ΔTPcmep, the power-off controlsection 47 outputs a power off signal Poff to the power supply circuit55. In response thereto, the power supply circuit 55 stops supply ofpower to all the circuits. The reason why wait time is provided beforepower off is to wait for mass estimation temperature TPma and coilestimation temperature TPco to decrease to some degree. Needless to say,during this wait period as well, estimation calculation for determiningmass estimation temperature TPma and coil estimation temperature TPco iscontinued. This operation improves accuracy in estimating massestimation temperature TPma and coil estimation temperature TPco in astate in which the ignition switch 35 is turned on next time.

Next, initial value calculation and estimation calculation for massestimation temperature TPma and coil estimation temperature TPco andoperation of limiting current of the brushless motor 11 will bedescribed in detail. The initial value calculation and estimationcalculation for mass estimation temperature TPma and coil estimationtemperature TPco are performed in the initial temperature correctioncalculation section 43. In the initial value calculation, ambienttemperature TPtr and printed board temperature TPbd at that time areused along with ambient temperature storage value TPtrep, printed boardtemperature storage value TPbdep, mass-ambient temperature differencestorage value ΔTPmtep, and coil-mass temperature difference storagevalue ΔTPcmep. Moreover, in initial value calculation for specialsituations, ambient temperature storage value TPtrep, mass temperaturestorage value TPmaep, and coil temperature storage value TPcoep areused. These storage values TPtrep, TPbdep, ΔTPmtep, ΔTPcmep, TPmaep, andTPcoep are stored in the EEPROM 60 as described above, and in responseto turn-on of the ignition switch 35, are read out by the dataread-write processing section 42 and supplied to the initial temperaturecorrection calculation section 43.

First, operation in an ordinary situation; i.e., a situation other thanthe below-described special situations, will be described. In this case,the initial temperature correction calculation section 43 firstsubtracts printed board temperature TPbd from printed board temperaturestorage value TPbdep to thereby obtain printed board temperature changeΔTPbd (=TPbdep−TPbd) between the time when the ignition switch 35 isturned off and the time when the ignition switch 35 is again turned on.

Subsequently, with reference to a mass temperature initial correctioncoefficient map and a coil temperature initial correction coefficientmap provided in the initial temperature correction calculation section43, the initial temperature correction calculation section 43 calculatesa mass temperature initial correction coefficient Ktpm and a coiltemperature initial correction coefficient Ktpc corresponding to theprinted board temperature change ΔTPbd. These mass temperature initialcorrection coefficient map and coil temperature initial correctioncoefficient map respectively represent changes in mass temperature andcoil temperature corresponding to change in printed board temperatureTPbd, and decrease as the printed board temperature change ΔTPbdincreases, as shown in FIGS. 5A and 5B, respectively. Notably, thesemaps can be obtained empirically or calculated from the relation betweenheat radiation of the printed board and heat radiation of the massportion and coils.

Initial values of mass estimation temperature TPma and coil estimationtemperature TPco are successively calculated in accordance with thefollowing Equations (1) and (2) which use the above-mentionedmass-ambient temperature difference storage value ΔTPmtep, coil-masstemperature difference storage value ΔTPcmep, mass temperature initialcorrection coefficient Ktpm, coil temperature initial correctioncoefficient Ktpc, and ambient temperature TPtr.TPma=Ktpm·ΔTPmtep+TPtr   (1)TPco=Ktpc·ΔTPcmep+TPma   (2)

The reason why calculations of respective initial values of massestimation temperature TPma and coil estimation temperature TPco areperformed separately, and estimation calculations for mass estimationtemperature TPma and coil estimation temperature TPco to be describedlater are performed separately is that the mass portion and the coilsdiffer from each other in terms of heat radiation and heat generationcharacteristics.

Moreover, in the present embodiment, the mass temperature initialcorrection coefficient Ktpm and the coil temperature initial correctioncoefficient Ktpc are calculated by making use of printed boardtemperature change ΔTPbd. However, the mass temperature initialcorrection coefficient Ktpm and the coil temperature initial correctioncoefficient Ktpc may be calculated by making use of ambient temperaturechange ΔTPtr. In this case, there are prepared a mass temperatureinitial correction coefficient map and a coil temperature initialcorrection coefficient map which respectively define mass temperatureinitial correction coefficient Ktpm and coil temperature initialcorrection coefficient Ktpc which change in accordance with change inambient temperature TPtr. Ambient temperature TPtr is subtracted fromambient temperature storage value TPtrep stored in the EEPROM 60 toobtain a change value ΔTPtr (=TPtrep−TPtr), and a mass temperatureinitial correction coefficient Ktpm and a coil temperature initialcorrection coefficient Ktpc corresponding to the change value ΔTPtr(=TPtrep−TPtr) are calculated with reference to the mass temperatureinitial correction coefficient map and the coil temperature initialcorrection coefficient map, respectively.

Next, calculation of initial values of mass estimation temperature TPmaand coil estimation temperature TPco in special situations will bedescribed. A first special situation is the case where data were failedto be written in the EEPROM 60 at the time of previous power off(immediately before time t5 of FIG. 3); that is, the case where previousambient temperature storage value TPtrep, printed board temperaturestorage value TPbdep, mass-ambient temperature difference storage valueΔTPmtep, and coil-mass temperature difference storage value ΔTPcmepcannot be read out of the EEPROM 60. In this situation, calculation ofthe above-mentioned Equations (1) and (2) is impossible. In this case,mass estimation temperature TPma and coil estimation temperature TPcoare initially set to the mass temperature storage value TPmaep and coiltemperature storage value TPcoep written in the EEPROM 60 at the time ofturn off of the ignition switch 35 (at time t3 of FIG. 3). Moreover, inthe case where even the mass temperature storage value TPmaep and thecoil temperature storage value TPcoep were not written in the EEPROM 60,the tentative data written in the EEPROM 60 at time t2 of FIG. 3 areutilized.

A second special situation is the case where printed board temperatureTPbd assumes an anomalous value. In this situation, calculation of themass temperature initial correction coefficient Ktpm and the coiltemperature initial correction coefficient Ktpc corresponding to theprinted board temperature change ΔTPbd is impossible, and calculation ofthe above-mentioned Equations (1) and (2) is impossible. In this case,as in the first special case, mass estimation temperature TPma and coilestimation temperature TPco are initially set to the mass temperaturestorage value TPmaep and coil temperature storage value TPcoep,respectively.

A third special situation is the case where ambient temperature TPtr isnot higher than a considerably low, predetermined temperature (e.g., notgreater than 0° C.). In this situation, ambient temperature TPtr isconsiderably low, and the temperatures of the housing, stator, and coilsof the brushless motor 11 are expected to be equal to the considerablylow ambient temperature TPtr. In this case, mass estimation temperatureTPma and coil estimation temperature TPco are initially set to theambient temperature TPtr.

A fourth special situation is the case where the vehicle is placed inthe blazing sun in summer. Existence of this situation is determined onthe basis of conditions such that ambient temperature TPtr is higherthan ambient temperature storage value TPtrep, printed board temperatureTPbd is not lower than a considerably high, predetermined temperature(e.g., not lower than 85° C.), or ambient temperature TPtr is not lowerthan a considerably high, predetermined temperature (e.g., not lowerthan 85° C.). In this case, mass estimation temperature TPma isinitially set to the higher one of ambient temperature TPtr and masstemperature storage value TPmaep. Also, coil estimation temperature TPcois initially set to the higher one of ambient temperature TPtr and coiltemperature storage value TPcoep.

The temperature estimation calculation section 44 calculates thetemperatures of the housing (or stator) and coils of the brushless motor11, which change with passage of time, while repeatedly updating theinitially set mass estimation temperature TPma and the initially setcoil estimation temperature TPco. In this temperature calculation,rotational speed RV and motor-current square value Ismt are utilized.The rotational speed RV is rotational speed of the brushless motor 11calculated in the speed conversion section 53. The motor-current squarevalue Ismt is equal to the sum Id²+Iq² of square values of two phasecurrents Id and Iq flowing through the brushless motor 11, the squarevalues being calculated in the motor-current square value calculationsection 45. Notably, the motor-current square value Ismt is proportionalto quantity of heat generated by current flowing through the coils ofthe brushless motor 11.

Estimation calculation processing that the temperature estimationcalculation section 44 performs to obtain mass estimation temperatureTPma and coil estimation temperature TPco is shown as amotor-temperature estimation program in the flowchart of FIG. 6. Thismotor-temperature estimation program consists of steps S10 to S38 and isrepeatedly executed at short time intervals (e.g., 80 milliseconds).After starting the execution of the program from step S10, thecalculation section 44 determines in step S12 whether a rotation flagRVF is “0.” This rotation flag RVF indicates that the brushless motor 11is currently rotating when its value is “1” and indicates that thebrushless motor 11 is currently stopped when its value is “0.” When thevalue of the rotation flag RVF is “0,” the calculation section 44 makesa “Yes” determination in step S12, and processing proceeds to step S14.In step S14, the calculation section 44 determines whether therotational speed RV is not less than a predetermined low speed RV2(e.g., 0.3 radian/sec). When the rotational speed RV is not less thanthe predetermined low speed RV2, the calculation section 44 makes a“Yes” determination in step S14, proceeds to step S16 so as to changethe rotation flag RVF to “1,” and then proceeds to step S18. When therotational speed RV is less than the predetermined low speed RV2, thecalculation section 44 makes a “No” determination in step S14, andprocessing proceeds to step S26.

In step S18, the calculation section 44 selects parameters for rotation(parameters in relation to rotation periods of the brushless motor 11)as parameters to be used in the below-described calculation processing.These parameters include current mass delay count Nima, current masstemperature correction coefficient Kima, current coil delay count Nico,and current coil temperature correction coefficient Kico.

Meanwhile, when the value of the rotation flag RVF is “1,” thecalculation section 44 makes a “No” determination in step S12, andprocessing proceeds to step S20. In step S20, the calculation section 44determines whether the rotational speed RV is not greater than apredetermined speed RV1 (e.g., 0.1 radian/sec) smaller than thepredetermined speed RV2. The reason why the predetermined speed RV1 isset smaller than the predetermined speed RV2 is to provide hysteresis indetection of rotating state and stopped state of the brushless motor 11to thereby avoid hunting. When the rotational speed RV is not greaterthan the predetermined speed RV1, the calculation section 44 makes a“Yes” determination in step S20, proceeds to step S22 so as to changethe rotation flag RVF to “0,” and then proceeds to step S24. When therotational speed RV is greater than the predetermined speed RV1, thecalculation section 44 makes a “No” determination in step S20, andprocessing proceeds to step S26.

In step S24, the calculation section 44 selects parameters for stoppage(parameters in relation to stop periods of the brushless motor 11) asparameters to be used in the below-described calculation processing.

As described above, the various parameters used in the calculationprocessing are set to different values for rotating periods and stopperiods of the brushless motor 11. Specifically, the current mass delaycount Nima and the current coil delay count Nico delay changes in valuesin estimation calculation for mass estimation temperature TPma and coilestimation temperature TPco. That is, these parameters smooth changes inthese values. The greater the values of the current mass delay countNima and the current coil delay count Nico, the greater the delay thatthe values represent. Therefore, during periods in which the brushlessmotor 11 is stopped, the current mass delay count Nima and the currentcoil delay count Nico are set to smaller values as compared with periodsin which the brushless motor 11 rotates. The current mass temperaturecorrection coefficient Kima and the current coil temperature correctioncoefficient Kico are parameters which represent the degree of influenceof current square value for mass Isma and current square value for coilsIsco on mass estimation temperature TPma and coil estimation temperatureTPco. The greater the values of the current mass temperature correctioncoefficient Kima and the current coil temperature correction coefficientKico, the greater the degree of influence that the values represent.Therefore, during periods in which the brushless motor 11 is stopped,the current mass temperature correction coefficient Kima and the currentcoil temperature correction coefficient Kico are set to larger values ascompared with periods in which the brushless motor 11 rotates. Notably,current square value for mass Isma and current square value for coilsIsco correspond to respective quantities of heat generation which causetemperature increases in the mass portion and the coils, and are valuesobtained through low-pass filtering of motor current square value Ismt.

After the processing of steps S12 to S24, in step S26, the calculationsection 44 of the motor overheat prevention control section 40 performslow-pass-filtering calculation of the following Equation (3), which usesmotor current square value Ismt and current mass delay count Niam, tothereby obtain current square value for mass Isma.Isma(n)=Isma (n−1)+{Ismt−Isma(n−1)}/Nima  (3)

Notably, in Equation (3), Isma(n) represents the present calculatedvalue (present value) of current square value for mass Isma, andIsma(n−1) represents the previous calculated value (value at time 80milliseconds before the present time) of current square value for massIsma (see FIG. 7). Subsequently, in step S28, the calculation section 44calculates mass temperature increase ΔTPma through execution ofcalculation according to the following Equation (4), which uses thecalculated current square value for mass Isma(n) and the current masstemperature correction coefficient Kima.ΔTPma=a·Kima·Isma(n)  (4)

Notably, in Equation (4), “a” is a predetermined proportionalityconstant. Subsequently, in step S30, the calculation section 44calculates mass estimation temperature TPma through execution ofcalculation according to the following Equation (5), which uses thecalculated mass temperature increase ΔTPma and ambient temperature TPtr.TPma=TPtr+ΔTPma  (5)

After the processing of steps S26 to S30, in step S32, the calculationsection 44 performs low-pass-filtering calculation of the followingEquation (6), which uses motor current square value Ismt and currentcoil delay count Nico, to thereby obtain current square value for coilsIsco.Isco(n)=Isco(n−1)+{Ismt−Isco(n−1)}/Nico  (6)

Notably, in Equation (6), Isco(n) represents the present calculatedvalue (present value) of current square value for coils Isco, andIsco(n−1) represents the previous calculated value (value at time 80milliseconds before the present time) of current square value for coilsIsco (see FIG. 7). Subsequently, in step S34, the calculation section 44calculates coil temperature increase ΔTPco through execution ofcalculation according to the following Equation (7), which uses thecalculated current square value for coils Isco(n) and the current coiltemperature correction coefficient Kico.ΔTPco=b·Kico·Isco(n)  (7).

Notably, in Equation (7), b is a predetermined proportionality constant.Subsequently, in step S36, the calculation section 44 calculates coilestimation temperature TPco through execution of calculation accordingto the following Equation (8), which uses the calculated coiltemperature increase ΔTPco and mass estimation temperature TPma. Afterthat, in step S38, the calculation section 44 ends the motor-temperatureestimation program.TPco=TPma+ΔTPco  (8)

As can be understood from the above description, temperatures of themass portion and coils of the brushless motor 11 are estimated indifferent manners of calculation between the case where the brushlessmotor 11 is rotating and the case where the brushless motor 11 isstopped; i.e., in consideration of difference in temperature increasecaused by current flowing through the coils between the case where thebrushless motor 11 is rotating and the case where the brushless motor 11is stopped. As a result, temperatures of the mass portion and coils ofthe brushless motor 11 can be detected by means of a simpleconfiguration, without necessity of incorporating a temperature sensorin the brushless motor 11. Moreover, in estimation of coil temperature,the mass portion and the coils which differ in heat radiation and heatgeneration characteristics are separated from each other, andcalculation is performed while different parameters are used for themass portion and the coils. Therefore, coil temperature can be detectedaccurately.

The value of coil estimation temperature TPco calculated in thetemperature estimation calculation section 44 is supplied to the firstcurrent limit value calculation section 46 a and the second currentlimit value calculation section 46 b. The first current limit valuecalculation section 46 a calculates a first current limit value IL1corresponding to the calculated value of coil estimation temperatureTPco with reference to a current limit value table. This current limitvalue table is provided in the first current limit value calculationsection 46 a, and, as shown in FIG. 8, the table stores the firstcurrent limit value IL1, which decreases with increase in coilestimation temperature TPco.

The second current limit value calculation section 46 b calculates asecond current limit value IL2 which supplements the first current limitvalue IL1 calculated in the first current limit value calculationsection 46 a and forcedly limits current when coil estimationtemperature TPco becomes considerably high. When coil estimationtemperature TPco exceeds the very high, predetermined temperature, thesecond current limit value calculation section 46 b sets a targetcurrent limit value ILtg to a predetermined small value, and repeatedlyexecutes the calculation (low-pass filtering processing) of thefollowing Equation (9) at predetermined short intervals.IL2(n)=IL2(n−1)+Kf1·(ILtg−IL2(n−1)  (9)

Notably, in Equation (9), IL2(n) represents the present calculated value(present value) of second current limit value IL2, and IL2(n−1)represents the previous calculated value (value at time 80 millisecondsbefore the present time) of second current limit value IL2. Further, thecoefficient Kf1 is a predetermined constant less than “1.” Throughexecution of calculation of Equation (9), the second current limit valueIL2 gradually decreases to the target current limit value ILtg when coilestimation temperature TPco exceeds the predetermined temperature.

Meanwhile, when coil estimation temperature TPco becomes lower than apredetermined temperature, the second current limit value calculationsection 46 b sets the target current limit value ILtg to a predeterminedvalue (e.g., 60 A), and repeatedly executes the calculation of Equation(9). As a result, the above-mentioned forced current limit is cancelled,and the second current limit value IL2 gradually increases to the targetcurrent limit value ILtg set to the predetermined current value.

The thus calculated first and second current limit values IL1 and IL2are supplied to the minimum value selection section 46 c. The minimumvalue selection section 46 c selects the smaller one of the first andsecond current limit values IL1 and IL2 to be used as a motor-currentlimit value ILm. The thus-determined motor-current limit value ILm issupplied to the current limit section 22. As described above, thecurrent limit section 22 limits three phase currents Iu, Iv, and Iwflowing through the brushless motor 11 in accordance with themotor-current limit value ILm.

Accordingly, when the coil temperature of the brushless motor 11increases, the current supplied to the brushless motor 11 is limited,whereby the coil temperature does not increase. This prevents breakageof insulation, which breakage would otherwise occur because of meltingof coating of the coils. Further, in addition to the first current limitvalue IL1, the second current limit value IL2 is taken intoconsideration, whereby increase in coil temperature of the brushlessmotor 11 is properly suppressed even in the case where the limit ofcurrent of the brushless motor 11 by means of the first current limitvalue IL1 results in an increase in coil temperature.

In the case where the second current limit value IL2 is employed, whencoil estimation temperature TPco exceeds the predetermined temperature,current flowing through the brushless motor 11 is forcedly limited tothe predetermined limit value or less. Accordingly, temperature increaseof the coils of the brushless motor 11 can be suppressed without fail.Moreover, when current flowing through the brushless motor 11 is limitedto the predetermined limit value or less, the limit value changesgradually. This operation avoids abrupt change in output of thebrushless motor 11.

In the above-described embodiment, a motor-current square value Ismt(=Id²+Iq²) is calculated from two phase currents Id and Iq, andtemperature of the entire coil of the brushless motor 11 is estimated onthe basis of the motor-current square value Ismt. Alternatively, coiltemperature may be estimated on a phase-by-phase fashion on the basis ofthree phase currents Iu, Iv, and Iw actually flowing through thebrushless motor 11. In this case, the motor overheat prevention controlsection 40 receives data indicative of three phase currents Iu, Iv, andIw of the brushless motor 11 detected by means of the current sensor 24.Subsequently, in place of the motor-current square value Ismt of theabove-described embodiment, the motor overheat prevention controlsection 40 calculates respective current square values Iu², Iv², and Iw²of the three phases, and performs, on the respective current squarevalues Iu², Iv², and Iw², the calculation processing applied to themotor-current square value Ismt of the above-described embodiment, tothereby estimate the respective coil temperatures of the individualphases. Subsequently, of the coil temperatures of the individual phases,the highest coil temperature is selected, and current flowing throughthe brushless motor 11 is limited on the basis of the selected coiltemperature.

In the above-described embodiment, mass estimation temperature TPma ofthe mass portion (housing, stator, etc.) is utilized in the course ofcalculation of coil estimation temperature TPco of the brushless motor11. Other use of mass estimation temperature TPma is not described.However, mass estimation temperature TPma may be used to correct adecrease in permeability of the stator caused by temperature increase ofthe brushless motor 11. In this case, three phase currents of thebrushless motor 11 are increased as mass estimation temperature TPmaincreases.

In the above-described embodiment, three phase currents Iu, Iv, and Iware detected by means of the current sensor 24. However, in analternative embodiment, only two phase currents are detected by means ofthe current sensor 24, and the remaining phase current is calculated byuse of the detected two phase currents. For example, of three phasecurrents Iu, Iv, and Iw, two phase currents Iu and Iv are detected bymeans of the current sensor 24, and the remaining phase current Iw iscalculated by the formula —(Iu+Iv).

In the present embodiment, the present invention is applied to thebrushless motor 11. However, the present invention can be applied tovarious types of motors other than the brushless motor 11.

In the present embodiment, the present invention is applied to anelectric motor for imparting assist torque to rotating operation of thesteering wheel 12 of the vehicle. However, the present invention can beapplied to other various types of electric motors mounted on thevehicle; for example, an electric motor which is incorporated in asteer-by-wire-type steering apparatus and imparts steering torque towheels. Moreover, the present invention can be applied to electricmotors incorporated in various apparatus other than vehicles.

The present invention is not limited to the above-described embodimentand modifications, and other various modifications may be employedwithin the scope of the present invention.

1. A motor-temperature estimation apparatus comprising: a currentdetection device for detecting current flowing through a coil of anelectric motor; a determination device for determining whether theelectric motor is in a rotating state or in a stopped state; and anestimation calculation device for estimatingly calculating a temperatureof the electric motor on the basis of the detected current and throughcalculation which changes depending on results of the determination asto whether the electric motor is in the rotating state or in the stoppedstate and considers a difference, in temperature increase caused bycurrent flowing through the coil, between the rotating state and thestopped state of the electric motor.
 2. A motor-temperature estimationapparatus according to claim 1, wherein the estimation calculationdevice comprises: a square value calculation device for calculating asquare value of the detected current; a low-pass-filtering device forperforming low-pass-filtering calculation for the calculated currentsquare value in a manner which changes depending on results of thedetermination as to whether the electric motor is in the rotating stateor in the stopped state; a temperature increase calculation device forcalculating a temperature increase stemming from current flowing throughthe coil on the basis of the low-pass-filtered current square value; anda temperature calculation device for calculating temperature of theelectric motor on the basis of the calculated temperature increase.
 3. Amotor-temperature estimation apparatus according to claim 2, wherein thelow-pass filtering device smoothes change in the calculated currentsquare value, and when the electric motor is in the stopped state, thelow-pass filtering device renders change in the calculated currentsquare value sharp, as compared with the case where the electric motoris in the rotating state.
 4. A motor-temperature estimation apparatusaccording to claim 2, wherein the temperature increase calculationdevice calculates a temperature increase which increases with thelow-pass-filtered current square value; and when the electric motor isin the stopped state, the temperature increase calculation deviceincreases the increase rate of the temperature increase, as comparedwith the case where the electric motor is in the rotating state.
 5. Amotor-temperature estimation apparatus according to claim 1, wherein theelectric motor is of a three-phase type; the current detection devicedetects two phase currents converted from three phase currents; and theestimation calculation device calculates and estimates temperature ofthe electric motor on the basis of the sum of respective square valuesof the two phase currents.
 6. A motor-temperature estimation apparatusaccording to claim 1, wherein the electric motor is of a three-phasetype; the current detection device detects three phase currents; and theestimation calculation device calculates and estimates temperature ofthe electric motor on the basis of respective square values of the threephase currents.
 7. A motor-temperature estimation apparatus according toclaim 1, wherein the electric motor is of a three-phase type; thecurrent detection device detects two phase currents, and calculates theremaining phase current from the detected two phase currents; and theestimation calculation device calculates and estimates temperature ofthe electric motor on the basis of respective square values of the threephase currents.
 8. A motor-temperature estimation apparatus comprising:an ambient temperature detection device for detecting ambienttemperature of an electric motor; a current detection device fordetecting current flowing through a coil of the electric motor; a squarevalue calculation device for calculating a square value of the detectedcurrent; a determination device for determining whether the electricmotor is in a rotating state or in a stopped state; a mass-temperatureincrease calculation device for calculating, on the basis of thecalculated current square value, a temperature increase of a massportion of the electric motor stemming from current flowing through thecoil, through calculation which changes depending on results of thedetermination as to whether the electric motor is in the rotating stateor in the stopped state and considers a difference, in temperatureincrease caused by current flowing through the coil, between therotating state and the stopped state of the electric motor; and amass-temperature calculation device for calculating temperature of themass portion of the electric motor by adding the calculated temperatureincrease of the mass portion to the detected ambient temperature.
 9. Amotor-temperature estimation apparatus according to claim 8, wherein themass-temperature increase calculation device comprises first low-passfiltering device for performing low-pass-filtering calculation for thecalculated current square value in a manner which changes depending onresults of the determination as to whether the electric motor is in therotating state or in the stopped state, wherein the temperature increaseof the mass portion of the electric motor stemming from current flowingthrough the coil is calculated on the basis of the low-pass-filteredcurrent square value.
 10. A motor-temperature estimation apparatusaccording to claim 9, wherein the first low-pass filtering devicesmoothes change in the calculated current square value, and when theelectric motor is in the stopped state, the first low-pass filteringdevice renders change in the calculated current square value sharp, ascompared with the case where the electric motor is in the rotatingstate.
 11. A motor-temperature estimation apparatus according to claim9, wherein the mass-temperature increase calculation device calculates atemperature increase of the mass portion which increases with thecurrent square value low-pass-filtered by device of the first low-passfiltering device; and when the electric motor is in the stopped state,the mass-temperature increase calculation device increases the increaserate of the temperature increase of the mass portion, as compared withthe case where the electric motor is in the rotating state.
 12. Amotor-temperature estimation apparatus according to claim 8, furthercomprising: a coil-temperature increase calculation device forcalculating, on the basis of the calculated current square value, atemperature increase of the coil of the electric motor stemming fromcurrent flowing through the coil, through calculation which changesdepending on results of the determination as to whether the electricmotor is in the rotating state or in the stopped state; and acoil-temperature calculation device for calculating temperature of thecoil of the electric motor by adding the calculated temperature increaseof the coil to the calculated temperature of the mass portion.
 13. Amotor-temperature estimation apparatus according to claim 12, whereinthe coil-temperature increase calculation device comprises secondlow-pass filtering device for performing low-pass-filtering calculationfor the calculated current square value in a manner which changesdepending on results of the determination as to whether the electricmotor is in the rotating state or in the stopped state, wherein thetemperature increase of the coil of the electric motor stemming fromcurrent flowing through the coil is calculated on the basis of thelow-pass-filtered current square value.
 14. A motor-temperatureestimation apparatus according to claim 13, wherein the second low-passfiltering device smoothes change in the calculated current square value,and when the electric motor is in the stopped state, the second low-passfiltering device renders change in the current square value sharp, ascompared with the case where the electric motor is in the rotatingstate.
 15. A motor-temperature estimation apparatus according to claim13, wherein the coil-temperature increase calculation device calculatesa temperature increase of the coil which increases with the currentsquare value low-pass-filtered by device of the second low-passfiltering device; and when the electric motor is in the stopped state,the coil-temperature increase calculation means device increases theincrease rate of the temperature increase of the coil, as compared withthe case where the electric motor is in the rotating state.
 16. Amotor-temperature estimation apparatus according to claim 1, wherein theelectric motor is incorporated in a steering apparatus of a vehicle. 17.A motor control apparatus comprising: a current detection device fordetecting current flowing through a coil of an electric motor; adetermination device for determining whether the electric motor is in arotating state or in a stopped state; a coil-temperature estimationcalculation device for estimatingly calculating temperature of the coilof the electric motor on the basis of the detected current and throughcalculation which changes depending on results of the determination asto whether the electric motor is in the rotating state or in the stoppedstate and considers a difference, in temperature increase caused bycurrent flowing through the coil, between the rotating state and thestopped state of the electric motor; and a current limiting device forlimiting current flowing through the electric motor in accordance withthe estimatingly calculated temperature of the coil.
 18. A motor controlapparatus comprising: an ambient temperature detection device fordetecting ambient temperature of an electric motor; a current detectiondevice for detecting current flowing through a coil of the electricmotor; a square value calculation device for calculating a square valueof the detected current; a determination device for determining whetherthe electric motor is in a rotating state or in a stopped state; amass-temperature increase calculation device for calculating, on thebasis of the calculated current square value, a temperature increase ofa mass portion of the electric motor stemming from current flowingthrough the coil, through calculation which changes depending on resultsof the determination as to whether the electric motor is in the rotatingstate or in the stopped state; a mass-temperature estimation device forestimatingly calculating temperature of the mass portion of the electricmotor by adding the calculated temperature increase of the mass portionto the detected ambient temperature; a coil-temperature increasecalculation device for calculating, on the basis of the calculatedcurrent square value, a temperature increase of the coil of the electricmotor stemming from current flowing through the coil, throughcalculation which changes depending on results of the determination asto whether the electric motor is in the rotating state or in the stoppedstate; a coil-temperature estimation device for estimatingly calculatingtemperature of the coil of the electric motor by adding the calculatedtemperature increase of the coil to the estimatingly calculatedtemperature of the mass portion; and a current limiting device forlimiting current flowing through the electric motor in accordance withthe estimatingly calculated temperature of the coil.
 19. A motor controlapparatus according to claim 17, wherein the current limit device limitscurrent flowing through the electric motor to a predetermined limitvalue or less, when the estimatingly calculated coil temperature exceedsa predetermined temperature.
 20. A motor control apparatus according toclaim 19, wherein when the current limit device limits the currentflowing through the electric motor to the predetermined limit value orless, the current limit device gradually changes the limit value.
 21. Amotor control apparatus according to claim 17, wherein the current limitdevice limits current flowing through the electric motor to a limitvalue or less, the limit value decreasing as the estimatingly calculatedtemperature of the coil increases.
 22. A motor control apparatusaccording to claim 17, wherein the electric motor is incorporated in asteering apparatus of a vehicle.